OPEC: operation-based security isolation for bare-metal embedded systems

Bare-metal embedded systems usually lack security isolation. Attackers can subvert the whole system with a single vulnerability. Previous research intends to enforce both privilege isolation (to run application code at the unprivileged level) and resource isolation for global variables and peripherals. However, it suffers from partition-time and execution-time over-privilege issues, due to the limited hardware resources (MPU regions) and the improper way to partition a program.

In this paper, we propose operation-based isolation for bare-metal embedded systems. An operation is a logically independent task composed of an entry function and all functions reachable from it. To solve the partition-time overprivilege issue, we utilize the global variables shadowing technique to reduce the needed MPU regions to confine the access of the global variables. To mitigate the executiontime over-privilege issue, we split programs into code compartments (called operation) that only contain necessary functions to perform specific tasks, thereby removing the resources needed by unnecessary functions. We implement a prototype called OPEC, which contains an LLVM-based compiler and a reference monitor. The compiler partitions a program and analyzes the resource dependency for each operation. With the hardware-supported privilege levels and MPU, the reference monitor is responsible for enforcing the privilege and resource isolation at runtime. Our evaluation shows that OPEC can achieve the security guarantees for the privilege and resource isolation with negligible runtime overhead (average 0.23%), moderate Flash overhead (average 1.79%), and acceptable SRAM overhead (average 5.35%).